68mm is still common but there are plenty of old frames which used oddball BBs. 68mm will probably be around for a good while yet, there is a push for something else, only time will tell if something sticks but I don't see 68mm going away anytime soon. The old brakes are starting to thin out and the selection of side pulls and the like are not what they once were, which could become an issue but we still have some good options there. Derailers are generally not a huge deal even if you have a frame that wants something weird.

Then we have the various headsets, some of those can be troublesome.

Single piece cranks and BBs are just about dead, as are cottered. We still have some options there but the quality is getting terrible and I suspect the only reason they are still made is because the machines still work but once they die that will be the end.

They’re somewhat less common indeed. A lot of manufacturers now like some proprietary variation of various pressfit standards.

You can still find bsa frames. Surly, as an example.